In practical application, there is a requirement that an authorized mission critical push to talk (MCPTT for short) user may need to initiate a group call involving multiple MCPTT groups. For example, in some emergencies, fire brigades and police forces cooperate with each other. Because an on-site commander may need to coordinate actions of multiple MCPTT groups to deliver a same instruction and task, a call involving the multiple MCPTT groups is initiated. In the prior art, such a function of a multi-group call cannot be supported yet.